Recently, the predicted depletion of conventional energy sources such as oil and coal has brought about an increasing interest in alternative energy sources. In particular, a fuel cell, as an energy storage system, is advantageous in that it is highly efficient, does not discharge pollutants such as NOx and SOx, and the fuel used is abundant, and thus attracts much attention.
A fuel cell is a power generation system which converts chemical reaction energy of a fuel and an oxidizing agent into electrical energy. Typically, hydrogen, methanol or hydrocarbons such as butane are used as the fuel and oxygen is used as the oxidizing agent.
The most basic unit to generate electricity in the fuel cell is a membrane electrode assembly (MEA), which is composed of an electrolyte membrane, and an anode and a cathode that are formed on both surfaces of the electrolyte membrane. Referring to FIG. 1 and Reaction Scheme I illustrating a mechanism via which a fuel cell generates electricity (reaction scheme of the fuel cell in the case where hydrogen is used as the fuel), at the anode, oxidation occurs to produce hydrogen ions and electrons and the hydrogen ions move through the electrolyte membrane to the cathode. At the cathode, oxygen (oxidizing agent) and hydrogen ions transferred through the electrolyte membrane react with electrons to produce water. Based on these reactions, electron transfer occurs in an external circuit.At anode electrode: H2→2H++2e−At cathode electrode: ½O2+2H++2e−→H2OOverall reaction: H2+½O2→H2Os  [Reaction Scheme I]
In this reaction, the polymer electrolyte membrane undergoes 15 to 30% of variation in membrane thickness and variation in volume depending on temperature and hydration, and in particular, undergoes 200% or more of variation in volume by 3 to 50% by weight of methanol fuels. Accordingly, electrolyte membranes undergo repeated swelling and contraction depending on operating conditions of fuel cells and polymer chains disentangle in polymer electrolyte membranes due to variation in volume, mechanical strength is reduced and fine pores or cracks occur.
These fine pores or cracks cause crossover of hydrogen or methanol crossover, thus resulting in deterioration in durability of fuel cells.
For this reason, a perfluorosulfonic acid resin membrane made of a perfluorosulfonic acid resin (trade name: Nafion) with superior conductivity, mechanical properties and chemical resistance is generally used as the polymer electrolyte membrane. However, the perfluorosulfonic acid resin is disadvantageously expensive, thus increasing manufacturing costs of fuel cells.
Therefore, interest in hydrocarbon electrolyte membranes that are cheaper than fluorine electrolyte membranes such as perfluorosulfonic acid resins is increasing. Hydrocarbon electrolyte membranes undergo relatively little deterioration in chemical resistance by-products produced by gas permeation during actual operation of fuel cells due to low gas permeation as compared to fluorine electrolyte membranes. However, general hydrocarbon electrolyte membranes undergo great variation in volume depending on variation in humidification conditions and are very fragile, and thus, disadvantageously, mechanical resistance during actual operation of fuel cells is difficult to secure. For example, hydrocarbon membranes are considerably vulnerable in cycle testing including repeated humidification and dehumidification that is a representative method for evaluating mechanical resistance of electrolyte membranes.
In addition, improvement of electrolyte membrane resins or filling a porous material with an electrolyte membrane resin is generally attempted to enhance durability of polymer electrolyte membranes for fuel cells. However, in a case in which strength of the electrolyte membrane increases, ion exchange capability is generally deteriorated, and the method of filling the porous material exhibits great improvement in resistance, but has problems of great difficulties associated with processes and increase in costs of raw materials. In particular, hydrocarbon electrolyte membrane resins undergo great variation in volume depending on variation in humidification conditions and thus cannot obtain significant improvement in resistance in spite of using the porous material. As another method, there is preparation by mixing an electrolyte membrane resin with a substance to improve resistance. This method disadvantageously impedes a mixing process and, in particular, does not exhibit remarkable effects.
Accordingly, attempts to solve these problems have been made in the art and the present invention has been developed under these technical circumstances.